well these questions are actually for two works in progress, one is a mech for the static mesh class that you will be able to get close to and the other is a helicopter that you can ride so the texture needs to look good on that too. but for the record the helicopter is actually pretty cool, I'd hate to ruin it by having a half ass texture on it.

Some of the built in meshes that need to be viewed close,use a few textures.For example look at BastienHardware Ramp02 it is 300*256 mesh and it uses 4 textures two of 512*512 one 1024*1024 and a 256*512 so would guess a few largish textures not to bad for fps.Do remember reading on unrealwiki site that a big texture was better than a few little ones .

I would advise going for multiple textures for another reason too. It would make it a lot easier for you to also ex-/import parts of the mech so you can e.g. easily use them in a workshop area of the map, a damaged one in a battlefield or something like this.